The present invention is directed to a sealing arrangement for centrifugal pumps and is especially effective when embodied in high energy centrifugal pumps used as feed pumps for nuclear reactor systems or the like.
The feed pumps utilized in electric generation systems typically handle liquids which are heated to as much as 550.degree. F. Such pumps are of the high energy type wherein the driving power is as great as 40,000 hp and the shaft speed may be as high as 8,000 rpm. The pump wheeel of such a pump is typically provided with a radially acting inner seal operating on an axially extending portion of the wheel adjacent the shaft to prevent axial leakage of pump fluid across the wheel.
Because of the high energy nature of the wheel, there must be such a clearance radially and axially about the wheel that the stationary portions to the pump define a chamber about the discharge portion thereof. This chamber must be sufficiently large to prevent the wheel from contacting the surrounding pump surfaces which contact would severely damage the wheel. Therefore, the chamber has a sufficient volume to permit the development of significant transient pressure differentials across the pump wheel.
The applicant has discovered that these transient pressure differentials are often sufficiently great to cause axial "shuttle" of the rotor which tends to accelerate the rotor against the thrust bearings which, in turn, give extra loading to the thrust bearings and acts to deflect the bearing brackets. In the case of high energy pumps, this action can be quite extensive at low flow rates.
Usually, such transient pressure differentials are developed across the pump wheel during the start-up period of the pump. When high energy pumps are utilized in systems such as nuclear reactor systems, requiring as long as six months to build up to normal flow rates, the phenomena of "shuttle" is especially prevalent.
The applicant herein has discovered that the shuttle phenomena in such high energy centrifugal pumps may be substantially reduced or even eliminated by a unique and a very uncomplicated arrangement wherein the pump may be axially restrained against shuttle without a rigid restraining means which, in itself, could cause damage to the wheel and other parts of the pump arrangement.